Plastic films and glass plates have fine projections on their surfaces, and when these are used as the panel substrates of liquid crystal devices, the projections hinder the uniformity of the gap between the panel substrates, causing display defects. For example, plastic films generally have projections of several .mu.m to over ten .mu.m in height on their surfaces. When the plastic films having such projections are used in TN (twisted nematic) cells or STN (super-twisted nematic) cells wherein liquid crystals are interposed between substrates arranged generally with a space of 6 to 10 .mu.m, the projections higher than the space cause considerable display defects.
Particularly, liquid crystal display devices using ferroelectric liquid crystals need substrates arranged with a space of about 2 .mu.m, and it is very difficult to produce liquid crystal display devices free from display defects by using such plastic film substrates or glass substrates.
When the electrode layers of electroded substrates are coated with an insulating film or the like, foreign matter or gel in the insulating layer tends to form projections on its surface so as to deteriorate the surface flatness. So when liquid crystals are sealed between the substrates arranged with a space of several microns, the projections also cause considerable display defects.
Japanese Patent Application Unexamined Publication No. 6-758 discloses a polishing apparatus for flattening the surfaces of the filter substrates of liquid crystal panels, by conveying an abrasive tape in one direction along the surfaces of rolls to give a pressing-polishing area, where a filter substrate is pressed to the abrasive tape at a uniform pressure while being put into reciprocating motion to polish the contacting portion. However, when polishing is carried out under a uniform pressure, the degree of polishing varies depending on not only the heights of the projections but also the forms thereof, and the heights of the polished projections cannot be adjusted accurately. Further, the pressure applied to the surface of the substrate makes the abrasive tape contact even the flat portions, so that when the substrate bears patterned transparent electrodes, the electrodes tend to be cut.
Japanese Patent Application Unexamined Publication No. 4-31030 discloses a method of producing heat resistant optical films having high surface flatness and good appearance by rotating an amorphous thermoplastic resin film of a glass transition temperature of 180.degree. C. or more under an applied pressure on an abrasive cloth fixed onto a stationary platform, with an abrasive liquid fed therebetween. The degree of polishing made by this method, however, also depends on the heights and forms of projections, and the heights of the polished projections cannot be adjusted accurately. Further, when substrates bearing patterned transparent electrodes are polished by this method, the electrodes tend to be broken because even the flat portions contact the abrasive cloth due to the pressure applied to the substrates.
A conventional method well known as laser repair, wherein only projections are removed by using laser beams or the like, is inefficient and lacks mass-productivity since the detection of projections is time-consuming and each projection is treated separately.